Recently a directly type backlight in which a plurality of cold-cathode tubes or LEDs (Light Emitting Diodes) are disposed is used to illuminate a large-size liquid crystal display television.
A diffuser plate having a strong light scattering property is used between an image display element and a light source such that the cold-cathode tube or LED is not visually recognized as the light source.
Because the diffuser plate diffuses light toward all directions by the light diffusion effect, the diffuser plate darkens the liquid crystal display apparatus. Usually a thickness of about 1 to 5 mm is necessary for the diffuser plate in order to enhance the light scattering property and to support an optical film adjoined on the diffuser plate. Therefore, a large amount of light is absorbed by the diffuser plate that darkens the liquid crystal screen display.
Conventionally, the diffuser plate used in the directly beneath type backlight is intended to diffuse the light emitted from the cold-cathode tube that is the light source to reduce brightness unevenness (lamp image). Accordingly, usually single or plural optical films are disposed on the diffuser plate in order to enhance the brightness in an observer-side direction.
Brightness Enhancement Film (BEF) (registered trademark, product of 3M in US) is widely used as the lens sheet in order to enhance the brightness of the liquid crystal display screen.
FIG. 14 is a sectional view schematically illustrating an example of disposition of a BEF, and FIG. 15 is a perspective view of the BEF. As illustrated in FIGS. 14 and 15, a BEF 185 is an optical film in which unit prisms 187 having triangular shapes in section are unidirectionally and periodically arrayed on a member 186. The unit prism 187 has a pitch larger than a wavelength of the light.
The BEF 185 collects the “off-axis” light, and redirects the light “on-axis” or recycles the light toward a viewer. That is, the BEF 185 can reduce the off-axis brightness to enhance the on-axis brightness during the use of the display (observation). As used herein, “on-axis” means a direction that is matched with a visual field direction F′ of the viewer, and usually “on-axis” is located on the side of the direction normal to a display screen.
In the use of the lens sheet typified by the BEF 185, a diffusion film (hereinafter referred to as lower diffusion film) in which a diffusion filler is applied onto a transparent base material to impart functions of diffusing and collecting the light is disposed between the diffuser plate and the lens sheet. Therefore, not only can the diffusion light output from the diffuser plate efficiently be collected, but also visibility of the light source, which is hardly eliminated only by the diffuser plate, can be suppressed.
When the light diffusion film is disposed between the lens sheet and the liquid crystal panel, not only can the side lobe be reduced, but also a moire interference fringe generated between the regularly-arrayed lenses and liquid crystal pixels can be prevented.
A display designer can use the BEF to achieve the desired on-axis brightness while reducing power consumption.
For example, Patent Documents 1 to 3 disclose a technique in which a brightness control member having a repetitive array structure of the prism typified by BEF is used in the display. In the optical sheet in which BEF is used as the brightness control member, the light emitted from the light source is finally output at a controlled angle from the film by refraction, so that the control can be performed so as to enhance the light intensity in a visual direction of the viewer.
[Patent Document 1] Japanese Patent Application Publication No. 1-37801
[Patent Document 2] Japanese Patent Application Laid-Open No. 6-102506
[Patent Document 3] Japanese PCT National Publication No. 10-506500
However, in the use of the BEF 185, sometimes a light component is vainly output in a lateral direction without going in the visual direction F′ of the viewer by reflection/refraction.
A line B of FIG. 16 expresses a characteristic of the BEF 185. In the line B, the light intensity is maximized at an angle of 0° (corresponding to the on-axis direction) with respect to the visual direction F′, and small light intensity peaks (side lobe) are generated near angles of ±90° with respect to the visual direction F′ to increase the light vainly output in the lateral direction.
The brightness distribution having such light intensity peaks is undesirable, and a smooth brightness distribution expressed by a line A of FIG. 16 is desirable, in which there is no light intensity peak near the angles of ±90°.
When only the on-axis brightness is excessively enhanced, a peak width of the brightness distribution curve is remarkably narrowed to extremely restrict a visual region. In order to properly spread the peak width, it is necessary to additionally use a light diffusion film that is different from the prism sheet, which causes a problem in that the number of members is increased.
There are strong market needs for the light weight, low power consumption, high brightness, and low profile in the liquid crystal display apparatus, and therefore the light weight, low power consumption, and high brightness are necessary for the backlight unit mounted on the liquid crystal display apparatus.
Particularly, in the color liquid crystal display apparatus continuing the remarkable development in recent years, because transmissivity of the liquid crystal panel is much lower than that of the monochrome liquid crystal panel, it is necessary to enhance the brightness of the backlight unit in order to achieve the low power consumption of the color liquid crystal display apparatus.
However, as described above, the conventional apparatus responds insufficiently to the needs for high brightness and low power consumption. Therefore, users still desire a backlight unit and display apparatus, which can implement the liquid crystal display apparatus of low cost, high brightness, high-quality display, and low power consumption.